An organic EL (electroluminescence) display apparatus has a large number of arrayed self luminous organic EL devices. The EL display apparatus does not require a backlight and does not have any viewing angle restrictions. Accordingly, it has been developed as a next generation display apparatus.
The organic EL device is a current light emitting device which can control luminance in response to an amount of current flow. Methods for driving the organic EL device include a simple-matrix method and an active-matrix method. The simple-matrix method allows a pixel circuit to be made simple, but it is difficult to achieve a large-sized and high definition display. For this reason, the active-matrix organic EL display apparatus, which has driving transistors for every pixel circuit, has been mainly used in recent years.
The driving transistor and its peripheral circuit are formed generally by TFT (Thin Film Transistors) made of poly-silicon or amorphous silicon. Although TFT has the disadvantage of a high threshold voltage fluctuation due to its low mobility, the TFT is suitable for a large-sized organic EL display apparatus because large sized TFT is easy to make and the cost of TFT is low. Further, a method for overcoming the disadvantage (fluctuation of the threshold voltage) has been studied by improving a pixel circuit. For example, Japanese Patent Application Publication JP2009-169145A1 describes an organic EL display apparatus which compensates the threshold voltage of the driving transistor.
The compensation of threshold voltage is performed as follows. First, a voltage larger than the threshold voltage is applied between a gate and source of the driving transistor in order to generate a current-flow in the driving transistor and to discharge a capacitor connected between the gate and the source of the driving transistor. The current in the driving transistor stops flowing when a terminal to terminal voltage of the capacitor (i.e. voltage between two terminals of the capacitor) decreases to the threshold voltage of the driving transistor. This terminal to terminal voltage is added to an image signal and an image is thereby displayed independently of the threshold voltage of the driving transistor.
If the terminal to terminal voltage of the capacitor is much higher than the threshold voltage, the capacitor is discharged rapidly because the current flowing in the driving transistor is large. However, as the terminal to terminal voltage of capacitor decreases toward the threshold voltage, the amount of current flowing in the driving transistor decreases. As a result, the discharging speed of the capacitor becomes slow. Thus, a long time is required before the terminal to terminal voltage of capacitor falls to the threshold voltage of the driving transistor. Practically, 10-100 micro-seconds, for example, may be required.
However, according to the pixel circuit and the driving method described in JP2009-169145A1, a data line for supplying an image signal is used for compensating the threshold voltage. This limits the time available for the writing operation, and makes it difficult to achieve a large-sized or high definition display apparatus having a large numbers of pixels.